These waves occupy a spread of frequencies extending from the high frequency range (HF) to the ultrahigh frequency range (UHF), the highest frequencies being capable of reaching approximately 2 gigahertz (GHz) if the dielectric fluid is a gas, and approximately 1.5 GHz if the fluid is a liquid.
For several years, measurements have commonly been performed in the HF and very high frequency (VHF) frequency ranges to detect the impulses of partial electric discharges in electrical equipment installed in a casing filled with a dielectric fluid (a liquid or a gas), but that technique presents drawbacks. Corona discharges taking place in the vicinity of the equipment generate high intensity electromagnetic waves in the same HF and VHF frequency ranges up to about 200 megahertz (MHz) and that significantly interferes with the measurements.
Thus, in order to improve the quality of such measurements, it is necessary to perform them in the UHF frequency range so as to be unaffected by disturbances due to corona discharges. U.S. Pat. No. 5,804,972 describes a device for detecting partial electric discharge impulses in gas-insulated metal-clad electrical equipment by means of a UHF antenna installed inside the equipment and having one electrode mounted on the casing of the equipment. The transfer function of such an antenna is relatively high and flat in a UHF frequency band extending typically from 300 MHz to 1.8 GHz.
No provision is made in that device to be able to remove the antenna from its housing without temporarily taking the equipment out of service, since removing the antenna requires sealing between the gas inside the casing and the outside air to be broken.
The possibility of removing such an antenna for detecting partial discharge impulses from its housing does exist elsewhere in the field of electrical equipment such as transformers placed in a tank filled with a dielectric liquid such as oil.
A partial discharge sensor capable of being inserted through an emptying valve as far as an opening in the tank is described in U.S. Pat. No. 6,323,655. Such a sensor includes an antenna, e.g. a spiral antenna, and it can be installed and withdrawn without it being necessary to take the transformer out of operation. It further includes a bushing for coupling to the emptying valve, with the antenna being mounted on a support that is capable of sliding in said bushing inside the emptying valve. Thus, the sensor can be used in succession on a plurality of identical valves occupying different positions of the tank of a piece of oil-insulated electrical equipment without it being necessary to interrupt operation of the equipment.
That antenna is of a size that is smaller that the diameter of the emptying valve so as to enable it to pass along the bore of the valve. It should be observed that the transfer function of the antenna is inversely proportional to frequency and varies in the same direction as valve diameter. Unfortunately, the valves commonly used for transformers in oil-insulated tanks possess bores of relatively small diameter, typically less than 50 millimeters (mm). The sensor can thus have a transfer function which remains satisfactory in the HF and VHF frequency ranges, but which becomes poor for UHF frequencies. This deterioration leads to values that are too small for providing an antenna signal that is genuinely useful, and also gives rise to a multitude of resonance peaks which make it impossible to use any antenna signal properly.
If the sensor is fitted with a spiral antenna, adapting it to ultrahigh frequencies requires a spiral of a diameter greater than the diameter of emptying valves that are commonly installed, which means that it is not possible to pass the sensor through the valve.